Optimizing
feedback latency

The general latency requirement for touch feedback is 30 milliseconds.
For direct feedback it is not always possible to achieve this, as in high
CPU load cases it may take more than 30 ms already before the pointer event
reaches the application.

However, you can also affect the latency in your application: the maximum
additional latency for direct feedback is as long as it takes for the longest
running RunL of any active object running in application’s
thread to execute. Hence keeping all RunL functions short
improves the feedback latency (and of course also improves the overall responsiveness
of the application).

Another way of improving the latency is optimizing redrawing in such a
way that only the necessary area of the screen is drawn.

For example, if there are 20 buttons in the application and one of them
is pressed down by a pointer event, then only that button should be redrawn
and not the whole screen. This optimization improves both direct and area
registry based feedback, because the window server cannot receive pointer
events from the touch driver in case it is performing a draw operation, and
unnecessarily massive draw operations can thus have significant effect on
the latency.

Finally, it is recommended that you trigger direct feedback in HandlePointerEventL as
the first thing before doing any other processing, such as redrawing.